The present invention relates to an oriented, heat shrinkable thermoplastic film.
xe2x80x9cSoft shrinkxe2x80x9d films are known in the art. These films offer relatively high free shrink combined with relatively low shrink tension. They are suitable for packaging articles, such as textiles and paper, that are sensitive to, or whose packaged appearance suffers from effects, such as distortion, of a film with too high a shrink tension.
An example of such films is poly(vinyl chloride) (hereinafter xe2x80x9cPVCxe2x80x9d) film. PVC film exhibits good elastic recovery, high modulus, and low shrink tension. Unfortunately, PVC film suffers from poor sealability, low free shrink, and poor tear resistance. It also carbonizes during a heat sealing process, releasing hydrogen chloride. The ensuing hydrochloric acid attacks and corrodes metal surfaces of equipment.
Polyolefinic packaging materials have been developed for soft shrink end-use. An example is the polyolefinic film supplied commercially by Cryovac, Inc. under the designation D-940(trademark). This film provides a commercially useful alternative to PVC film, without the disadvantages of a chlorine-containing material. However, it would be desirable to provide a chlorine-free film that avoids the disadvantages of PVC film, but offers higher modulus and better elastic recovery than that now available from D-940, while maintaining good heat seal quality, high free shrink and a low shrink tension. Additionally, it would be advantageous to provide such attributes in a chlorine-free film having better optical quality (low haze) than is found in several present low shrink force chlorine-free films.
In a first aspect, a multilayer solid state oriented heat shrinkable film comprises a core layer comprising a linear random alpha-olefin/vinyl aromatic copolymer; and a first and second outer layer each comprising an olefinic polymer; wherein the film has a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal and transverse directions; a shrink tension of between 50 and 400 pounds per square inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In a second aspect, a multilayer solid state oriented heat shrinkable film comprises a core layer comprising a linear random alpha-olefin/vinyl aromatic copolymer; a first and second intermediate layer each comprising an ethylene copolymer having a melt index less than 2.0; and a first and second outer layer each comprising an olefinic polymer; wherein the film has a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal and transverse directions; a shrink tension of between 50 and 400 pounds per square inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In a third aspect, a multilayer oriented heat shrinkable film comprises a core layer comprising an ethylene copolymer having a melt index less than 2.0; a first and second intermediate layer each comprising a linear random alpha-olefin/vinyl aromatic copolymer; and a first and second outer layer each comprising an olefinic polymer; wherein the film has a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal direction and transverse direction; a shrink tension of between 50 and 400 pounds per square inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In a fourth aspect, a method of making a film comprises extruding a sheet of film comprising a core layer comprising a linear random alpha-olefin/vinyl aromatic copolymer; and a first and second outer layer each comprising an olefinic polymer; quenching the extruded sheet of film; reheating the quenched sheet of film to its orientation temperature; and orienting the reheated sheet of film to produce a heat shrinkable film, the film having a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal and transverse directions; a shrink tension of between 50 and 400 pounds per square, inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In a fifth aspect, a method of making a film comprises extruding a sheet of film comprising a core layer comprising a linear random alpha-olefin/vinyl aromatic copolymer; a first and second intermediate layer each comprising an ethylene copolymer having a melt index less than 2.0; and a first and second outer layer each comprising an olefinic polymer; quenching the extruded sheet of film; reheating the quenched sheet of film to its orientation temperature; and orienting the reheated sheet of film to produce a heat shrinkable film, the film having a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal and transverse directions; a shrink tension of between 50 and 400 pounds per square inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In a sixth aspect, a method of making a film comprising extruding a sheet of film comprising a core layer comprising an ethylene copolymer having a melt index less than 2.0; a first and second intermediate layer each comprising a linear random alpha-olefin/vinyl aromatic copolymer; and a first and second outer layer each comprising an olefinic polymer; quenching the extruded sheet of film; reheating the quenched sheet of film to its orientation temperature; and orienting the reheated sheet of film to produce a heat shrinkable film, the film having a Young""s modulus of between 50,000 and 200,000 pounds per square inch in at least one of the longitudinal and transverse directions; a free shrink of between 40% and 80% at 240xc2x0 F. in at least one of the longitudinal and transverse directions; a shrink tension of between 50 and 400 pounds per square inch, or a shrink force of between 20 and 180 grams, at 240xc2x0 F. in at least one of the longitudinal and transverse directions; and a haze of less than 6.
In any of the above-disclosed methods, the quenched extruded sheet of film can optionally be irradiated before the reheating step.
The reheated sheet of film can be oriented by e.g. trapped bubble orientation or tenter frame orientation.
xe2x80x9cAlpha-olefinxe2x80x9d herein refers to olefinic compounds, whether unsubstituted or substituted, in which the first two carbon atoms in the chain have a double bond therebetween. Examples include ethylene, propylene, hexene, and octene.
xe2x80x9cAlpha-olefin/vinyl aromatic copolymerxe2x80x9d (AO/VA) herein refers to homogeneous copolymers made from an alpha-olefinic monomer having from two to eight carbon atoms per molecule, and a vinyl aromatic monomer. An example is ethylene/styrene copolymer. The alpha-olefin/vinyl aromatic copolymer herein is a random linear copolymer and includes a pseudo-random copolymer. Although the present description primarily focuses on olefin/vinyl aromatic copolymers, terpolymers are also within the scope of such term. Terpolymers include ethylene/octene/styrene, ethylene/styrene/butene, propylene/styrene/ethylene, and ethylene/styrene/hexene terpolymers. Olefin/vinyl aromatic copolymers typically are prepared using single-site catalysts, especially using metallocene catalysts. Techniques for manufacturing AO/VA materials such as ethylene/styrene copolymer are disclosed in U.S. Pat. No. 5,658,625 (Bradfute et al.) and U.S. Pat. No. 5,703,187 (Timmers), all incorporated herein by reference in their entirety.
xe2x80x9cCore layerxe2x80x9d herein refers to an internal layer of a multilayer film, i.e. a layer bounded on both of its major surfaces by another layer or layers; in a film with an odd number of total layers, the core layer is typically the central layer of the film.
xe2x80x9cComp. Ex.xe2x80x9d, herein refers to xe2x80x9cComparative Examplexe2x80x9d. xe2x80x9cEx.xe2x80x9d herein refers to an example of the invention.
xe2x80x9cEthylene/alpha-olefin copolymerxe2x80x9d (EAO) herein refers to a copolymer of ethylene with one or more aliphatic comonomers selected from C3 to C10 alpha-olefins such as propene, butene-1,hexene-1, octene-1, etc. in which the molecules of the copolymers assemble as long polymer chains with relatively few short chain branches arising from the alpha-olefin which was reacted with ethylene. This molecular structure is to be contrasted with conventional high pressure low density (LDPE) or medium density polyethylenes which are highly branched homopolymers and contain both long chain and short chain branches. EAO includes such heterogeneous materials as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE), such as DOWLEX(trademark) or ATTANE(trademark) resins supplied by Dow, and ESCORENE(trademark) resins supplied by Exxon.
xe2x80x9cFree Shrinkxe2x80x9d values herein are in accordance with ASTM D 2732.
xe2x80x9cHazexe2x80x9d values herein are in accordance with ASTM D 1003.
xe2x80x9cHomogeneous ethylene/alpha olefin copolymerxe2x80x9d (HEAO) herein refers polymerization reaction products of narrow molecular weight distribution (Mw/Mn less than 3) and narrow composition distribution, referred to as to single-site polymerized polymers. These include linear homogeneous ethylene/alpha olefin copolymers (linHEAO) such as TAFMER(trademark) resins supplied by Mitsui Petrochemical Corporation, EXACT(trademark) resins supplied by Exxon, as well as long chain branched (IcbHEAO) AFFINITY(trademark) resins supplied by the Dow Chemical Company, or ENGAGE(trademark) resins supplied by DuPont Dow Elastomers. Homogeneous EAO copolymers may be polymerized using vanadium catalysts, as in the case of the TAFMER(trademark) products, or may employ a metallocene catalyst as in the case of the more recent EXACT(trademark) or AFFINITY(trademark) products.
xe2x80x9cHeterogeneousxe2x80x9d polymers herein refers to polymerization reaction products of relatively broad molecular weight and relatively wide composition distribution, such as VLDPE, LLDPE.
xe2x80x9cHigh density polyethylenexe2x80x9d (HDPE) herein refers to a polyethylene having a density of between 0.94 and 0.965 grams per cubic centimeter.
xe2x80x9cIntermediatexe2x80x9d herein refers to a layer of a multi-layer film which is between an outer layer and a core layer of the film.
xe2x80x9cInterpolymerxe2x80x9d herein refers to polymers assembled using two or more different monomers, and includes copolymers, terpolymers, tetrapolymers, etc. (for example ethylene/styrene interpolymer) The term may be used alternatively with the synonym xe2x80x9ccopolymerxe2x80x9d.
xe2x80x9cLaminationxe2x80x9d, xe2x80x9claminated sheetxe2x80x9d, and the like refer herein to the process, and resulting product, made by bonding together two or more layers of film or other materials. Lamination can be accomplished by joining layers with adhesives, joining with heat and pressure, and even spread coating and extrusion coating. The term laminate as used herein is also inclusive of coextruded multilayer films comprising one or more tie layers.
xe2x80x9cLxe2x80x9d and xe2x80x9cLDxe2x80x9d herein refer to the longitudinal direction, i.e. the direction of the film parallel to the path of extrusion. xe2x80x9cTxe2x80x9d and xe2x80x9cTDxe2x80x9d herein refer to the transverse direction, i.e. the direction of the film transverse to the path of extrusion.
xe2x80x9cLinear low density polyethylenexe2x80x9d (LLDPE) herein refers to polyethylene having a density between 0.916 and 0.925 grams per cubic centimeter.
xe2x80x9cLinear medium density polyethylenexe2x80x9d (LMDPE) herein refers to polyethylene having a density between 0.926 and 0.939 grams per cubic centimeter.
xe2x80x9cMelt indexxe2x80x9d herein is with reference to ASTM D 1238-90, Condition 190/2.16.
xe2x80x9cMelt-state orientedxe2x80x9d herein refers to films obtained by extruding polymer resins and immediately stretching the film, especially as a tube, then cooling with air to form the final film product. Most notable are the xe2x80x9cblown filmsxe2x80x9d wherein forced air is applied to the inner volume of a tube of emerging extrudate to stretch while in the molten state. Melt-state oriented blown films do not exhibit significant shrink until heated to a temperature approaching the melting point of at least one of the resins.
xe2x80x9cMulticomponent ethylene/alpha-olefin interpenetrating network resinxe2x80x9d or xe2x80x9cIPN resinxe2x80x9d herein refers to multicomponent molecular mixtures of polymer chains which are interlaced at a molecular level and are thus true solid state solutions. These become new compositions exhibiting properties distinct from parent constituents. IPN resins provide phase co-continuity leading to enhancement of physical properties, and may exhibit bimodal or multimodal curves when analyzed using TREF or CRYSTAF. xe2x80x9cIPN resinsxe2x80x9d includes semi-interpenetrating networks including crosslinked and uncrosslinked multicomponent molecular mixtures having a low density fraction and a high density fraction. Examples of IPN resins include ELITE(trademark) resins from Dow.
xe2x80x9cOuter layerxe2x80x9d herein refers to what is typically an outermost, usually surface layer or skin layer of a multi-layer film, although additional layers, coatings, and/or films can be adhered to it.
xe2x80x9cPolymerxe2x80x9d herein refers to homopolymer, copolymer, terpolymer, etc. xe2x80x9cCopolymerxe2x80x9d herein includes copolymer, terpolymer, etc.
xe2x80x9cShrink Tensionxe2x80x9d and xe2x80x9cShrink Forcexe2x80x9d herein refer to a force exerted by a one inch strip of film as a result of heat shrinking at a specified temperature. Testing is conducted except as specified, according to ASTM 2838. A 2.8 inch by 1 inch test strip (2.8 inches is the distance between the jaws of the strain gauge) is immersed in an oil bath (Dow Corning 200 silicone oil, 20 centistroke) preheated to 100xc2x0 F. [38xc2x0 C.] which is thereafter heated at a rate of approximately 10xc2x0 F. per minute to about 300xc2x0 F., or to the melting point of the specimen, while restraining the immersed specimen in the jaws of a strain gauge. Strain force is measured continuously and reported at 10xc2x0 increments and converted to psi by use of the initial thickness of the one-inch test strip.
xe2x80x9cSolid state orientedxe2x80x9d herein refers to films obtained by either co-extrusion or extrusion coating of the resins of different layers to obtain a primary thick sheet or tube (primary tape) that is quickly cooled to a solid state to quench (stop or slow) crystallization of the polymers, thereby providing a solid primary film sheet. The primary sheet is then reheated to the so-called orientation temperature, and thereafter biaxially stretched at the orientation temperature using either a tubular solid-state orientation process (for example a trapped bubble method) or using a flat solid-state orientation process (for example a simultaneous or sequential tenter frame), and finally rapidly cooled below the orientation temperature to provide a heat shrinkable film. In the trapped bubble solid state orientation process, the primary tape is stretched in the transverse direction (TD) by passing over an air bubble which is held between two rotating nip rolls, as well as stretched in the longitudinal direction (LD) by the differential speed between the two sets of nip rolls that contain the bubble. In the tenter frame process, the sheet or primary tape is stretched in the longitudinal direction by accelerating the sheet forward, while simultaneously or sequentially accelerating the sheet in the transverse direction by guiding the heat softened sheet through a diverging geometry frame. This tenter process typically refers to a flat sheet of relatively thick film. Solid state oriented films exhibit high free shrink when reheated to their orientation temperature.
xe2x80x9cStyrenic Rubberxe2x80x9d herein refers to thermoplastic rubbers based on styrene block copolymers in which for example anionic chemistry is employed to render long sequences (xe2x80x9cblocksxe2x80x9d) of styrene monomer alternating with long sequences of mono-olefinic and diolefinic monomers. Examples of such rubbers include styrene-butadiene-styrene block interpolymer (SBS), styrene-ethylene-butene-styrene block interpolymer (SEBS), and styrene-isoprene-styrene interpolymer (SIS). Such elastomers are typically star shaped, dendritic or linear, depending on the type of initiator. Examples of thermoplastic rubber resins include KRATON(trademark), K-RESIN(trademark), VECTOR(trademark) and STYROLUX(trademark).
xe2x80x9cSyndiotactic polystyrenexe2x80x9d herein refers to crystalline syndio-specific styrene polymers. Homopolymers are high melting resins having melting points in the range of about 270xc2x0 C. Such resins are sold by Dow Chemical Co. as QUESTRA(trademark) or from Idemitsu as XAREC(copyright). Copolymers exhibit somewhat lower melting points.
xe2x80x9cVinyl aromaticxe2x80x9d herein includes e.g. styrene, p-methyl styrene, vinyl toluene, vinyl xylene, vinyl naphthalene, and vinyl anthracene, with or without one or more substituents (for hydrogens) present on the aromatic ring(s), and/or the olefin carbon connected to the aromatic ring.
xe2x80x9cYoung""s modulusxe2x80x9d or xe2x80x9cmodulusxe2x80x9d values herein are in accordance with ASTM D 882.
All compositional percentages, including monomer percentages, used herein are presented on a xe2x80x9cby weightxe2x80x9d basis, unless designated otherwise. All film and sheet thicknesses designated in percentages are by percentage of total thickness of the film or sheet.